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The paper deals with a determination of the terminal settling velocity of coarse particles in quiescent visco-plastic liquids of Herschel-Bulkley type. Experiments on laminar settling of glass beads of different sizes were conducted in transparent Carbopol solutions of various rheological properties in a sedimentation column. The terminal settling velocity of a solitude bead was determined together with the rheological parameters of the Carbopol liquid. An evaluation of the experimental results confirms the existence of the laminar regime for all tests and compares the measured velocities with predictions by Wilson et al. method. Furthermore, an alternative method is proposed for a prediction of the terminal settling velocity in the laminar regime which uses a particle-based determination of the strain rate in the expression for the equivalent viscosity. A comparison with our experimental results shows that the predictions using the proposed method agree well with the experiments and the proposed method is in the laminar settling regime more accurate than the Wilson et al. method.

Abstract: The study of the settling behaviour of particles in viscoplastic fluids is closely related to the study of rheology. In this paper, a thorough examination of the flow behaviour of viscoplastic fluids, in the form of aqueous polyacrylamide solutions, has been presented. The results of this study suggest that the experimental fluids exhibit time-dependent flow characteristics, where the apparent viscosity of the solutions depends highly on their shear history. This time dependency has been attributed towards the processes of destruction and rejuvenation in the ‘structural network’ of the fluids (due to the presence of hydrogen bonding between polyacrylamide and water molecules), as they are subjected to changing rates of shear. A new fluid model was thus developed to capture this flow behaviour. This model, termed as ‘semi-viscoplastic’, features temporary yield stress characteristics that tend to dissipate once the structural network of the fluid is destroyed due to the application of shear. The time dependency of the fluid viscous parameters becomes apparent in the settling sphere experiment, where it has been demonstrated that a sphere that is following the flow path of another sphere tends to attain a fall velocity that is significantly higher than the preceding sphere. Based on this finding, a new generalised correlation has been developed, through which predictions of the fall velocity of spherical particles settling through viscoplastic fluids, of various shear history, can be made. [ABSTRACT FROM AUTHOR]

To reduce the cooling time in soft tooling process, one of the possible solutions is the use of composite mold materials, but that may affect melt mold flow properties. Therefore, a study on equivalent viscosity of melt mold material, which primarily influences the flow ability is essential. In this work, we have carried out an experimental study on equivalent viscosity of flexible mold materials (such as polyurethane and silicone rubber, which are of particular type) reinforced with highly thermal conductive filler particles, namely, aluminum and graphite powder. It has been observed that in addition to an increase of equivalent viscosity, different curing behaviors were noticed in mold materials reinforced with different fillers. By analyzing the performances of various equivalent viscosity models reported in literature, it has been observed that for higher particle size, the existing models deviate much from the experimental results. We have proposed an extension of the generalized model of Arefinia and Shojaei by including a factor that depends on particle size. It is found that the extension model provides better explanations compared to other models to the experimental results, especially for suspensions of flexible mold materials with higher particle sizes. Finally, a predictive approach is suggested for the equivalent viscosity of reinforced flexible mold materials, which may be useful to decide the amount of typical filler particles to be considered for mixing with a flexible mold material. [ABSTRACT FROM AUTHOR]

Abstract: A numerical study on the oil concentration effect of O/W emulsion in cold rolling operating in the mixed film lubrication regime has been carried out. The developed scheme is able to calculate oil concentration at any point within the inlet zone (IZ) and work zone (WZ), rolling pressure, film thickness, and contact ratio for various rolling speeds. Hence the intertwined effects of oil concentration of the supplied emulsion and rolling speed on strip rolling are discussed. The study encompasses mixed film regime with speeds S range from 10−5 to 10−3 and supplied emulsion''s oil concentration levels λ ds range from 5% to 90%. The result shows that a moderate rise in oil concentration occurs in the IZ followed by a rapid one at the beginning of the workzone. In most cases, the oil in the emulsion would have been transformed from disperse phase to continuous phase throughout the WZ. Notwithstanding further concentration, which depends on the oil concentration of the supplied emulsion, could still occur in the WZ. The effect of the concentration process is predominantly seen in the development of the lubricant pressure whilst its effect on the total pressure is less pronounced. The analysis of the results suggests that it is possible to lower the emulsion oil concentration without detrimental effects on the rolling process; and from the analysis of the outlet film thickness, it is shown that the variation of emulsions’ oil concentration could control the exit lubricant film thickness and consequently the strip surface quality. [Copyright &y& Elsevier]

Abstract: One of the factors contributing to the uncertainties involved in the estimation of particle settling velocity in viscoplastic fluids is the time-dependent effect where the viscous parameters of the fluid change as a particle flows through and shears the medium. These changes, particularly at low shear Reynolds numbers, are reflected in the settling velocity of a following sphere that is released some time after an initial one, with the following sphere having a significantly greater velocity. This study found that changes in both fall velocity and equivalent viscosity can be correlated satisfactorily by a power law equation to the dimensionless form of the time interval between releases, and the rheogram shape factor for the fluid. A collision of particles occurs in cases where the time interval between releases is small, after which the particles combine and travel at a terminal velocity. A new variable, β, which takes into account the different surficial stress of the combined spheres, was introduced to the correlation of Wilson et al. [Wilson, K.C., Horsley, R.R., Kealy, T., Reizes, J.A., Horsley, M.R., 2003. Direct prediction of fall velocities in non-Newtonian materials. Int. J. Miner. Process. 71, 17–30] β was found to depend on the rheogram shape factor for the fluid and the shear Reynolds number for the particle. The validity of this approach was supported by experimental data. [Copyright &y& Elsevier]

The estimation of particle fall velocities in viscoplastic materials is still full of uncertainties, despite the number of experimental studies and data correlations performed over the years by various authors. In addition to increasing the corpus of experimental data, the present paper develops a new method for direct (non-iterative) prediction of these velocities. Existing techniques employ the customary curve of drag coefficient versus Reynolds number, which has the major disadvantage that the required fall velocity occurs in both co-ordinates. A new curve (which can be applied to both Newtonian and non-Newtonian fluids) has been devised to overcome this difficulty, and thus eliminate the need for iteration. For non-Newtonian materials the new curve is used with an ‘equivalent Newtonian viscosity’, which has been calibrated against available experimental data. It is shown how this viscosity can be determined directly from the rheogram, without the need for a specific fit equation. [Copyright &y& Elsevier]

This paper presents a formulation of point contact elastohydrodynamic lubrication analysis for an isothermal, non-Newtonian flow. A coordinate system of the pressure gradient was employed herein. A Couette flow and a Poiseuille flow were considered along the directions of the zero and non-zero pressure gradients, respectively. The Poiseuille flow velocity was assumed to be represented by a 4th-order polynomial of z along the film thickness direction. The Couette flow velocity was assumed to be represented by a linear function of z. Subsequently, the modified Reynolds equation, which contains an equivalent viscosity, was obtained. Using Bauer's rheological model, the formulation presented in this study was applied to a grease that has been previously experimented upon. The results of previous studies were compared with those of the present study and a reasonable agreement was noted. The distribution of the equivalent viscosity showed a notable difference from that of Newtonian flow. The formulation can be incorporated easily to the usual elastohydrodynamic lubrication calculation procedure for Newtonian flow. The method can be easily applied to other non-Newtonian rheological models. The equivalent viscosity can be calculated using the one-parameter Newton-Raphson's method; as a result, the calculation can be performed within a reasonable time. [ABSTRACT FROM AUTHOR]